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Title: Soluble poly(ethylene glycol)-supported nitroxyl radicals: recyclable catalysts for the selective oxidation of alcohols
Author: Ferreira, Pedro M. S.
Awarding Body: University of Reading
Current Institution: University of Reading
Date of Award: 2004
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Abstract:
Nitroxyl radicals such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) are highly active catalysts for the selective oxidation of alcohols by mild oXida,nts such as hypochlorite (household bleach) or molecular oxygen. In this study, tethering of TEMPO units onto linear poly(ethylene glycol) (PEG) has afforded macromolecular catalysts that exhibit solubility in both aqueous and orQanic solvents. A range of linear 'linker-less' catalysts was obtained by direct tethering of the TEMPO moiety. onto 'the PEG chain end. A series of 'branched' catalysts were obtained by attachment of two TEMPO units on to the PEG chain end via a 5-hydroxy-isophthalic acid linker. Recovery of the dissolved polymer-supported catalysts was carried out by precipitation with a suitable solvent such as diethyl ether. The high catalyst activities and selectivities associated traditionally with nitroxyl-mediated hypochlorite oxidations of alcohols were retained by the 'linker-less' PEG-TEMPO catalysts. Although the selectivity remained unaltered, upon recycling, the 'linker-less' polymer-supported catalysts required extended reaction times in order to maintain high yields of the desired carbonyl compounds. Under identical conditions, the 'branched' catalysts exhibited catalytic activities up to ten times greater than the 'linker-less' system~·and remarkably were five times more active than 4-methoxy-TEMPO. In addition, minimal decreases in catalytic activity\were observed upon recycling of these 'branched' macromolecular catalysts via solvent-induced precipitation. The high catalytic activities and preservation of activity upon recycling of these branched systems was attributed to enhanced regeneration of the nitroxyl species as a result of intramolecular syn-proportionation. In the case of aerobic oxidations, the PEG-supported nitroxyl radical catalysts could be recycled easily and no drop of activity was observed for the recycled catalysts.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.485363  DOI: Not available
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